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168.   Fields of Force.  If a particle experiences a fore when placed at any point of a region the region is called &fiel of force.   The gravitational field of the earth, the electrics field of a charged body, and the magnetic field of a magne are examples of fields of force.
169.   Potential Energy and Fields of Force.  The potentis energy of a system is due to the overlapping of the field of force of its parts.    For instance, the earth and the moo: are not connected by anything material, yet they form system which has potential energy, because they are in eac other's gravitational field of force.    The fact that a stretche elastic string has potential energy seems to contradict thi statement, but this contradiction is only apparent.    Th potential energy of the stretched string is also due to th overlapping of the fields of force of its parts.    In this cas< however, molecules form the parts of the system.
170.   The Principle of the Degradation of Potential Energy  Consider a body which is displaced under the action of th forces of a field of force.    A certain amount of work is don during the displacement.    If the body is not acted upon b forces which are external to the field, then by the princip] of the conservation of energy, the energy of the body remain constant during the displacement.    Therefore the amour lost by one form of the energy of the body is gained by th other.    The work done is the measure of the amount of th energy transformed.
The principle of the conservation of energy does not thro^ any light on the question, " Which form of energy is th